The Assign option: "Assigns a new ID to unique records that need one, or assigns a new ID to all members of a group that don't have an ID. In addition, the assign operation copies and existing ID when a member of a match group already has an ID..."
The documentation does contain more detail on all of the options available using the Unique ID functionality of the Match transform and the options that you have as far as how the ID is assigned.
The starting number for the Unique id is based on how you configure the transform. If you are using the table1 as part of match, the unique id can keep the number assigned. Otherwise it will start numbering them based on the option used. The transform is configurable to meet your needs. Please refer to the designer guide or the content sensitive help in the match transform for additional information on all the options you have for creating a Unique ID.